Power transients were initiated in the HIFAR Materials Testing Reactor by altering both the temperature of the primary coolant circuit and the flow rate of the primary coolant. Analysis of the time variation of the reactor power and the D2O temperature provided information about the flow of D2O in the reactor aluminium tank and the time response of the negative power coefficient. The experimental power transients were compared with calculations carried out with the HIDYN code, which combines the neutron kinetics with the primary coolant characteristics.